Fig.: to keep quiet; to refrain from saying something you may regret

My in-laws kept going on about how happy was their boy, my husband, when he lived with them, how much he liked their house, how much he liked his mother's cooking and so on. I held my tongue until I could, then I told them they could have him back, if he wanted to go, that is

I miei suoceri continuavano a ripetermi quanto fosse felice il loro bambino, mio marito, quando viveva con loro, quanto gli piacesse la loro casa, quanto gli piacesse la cucina della mamma e via dicendo. Ho tenuto a freno la lingua finché ho potuto, poi ho detto loro che potevano anche riprenderselo, sempre che volesse andarsene

To hold is used in the sense of “refrain”, so if you refrain your tongue, you keep it from talking.
Geoffrey Chaucer used this idiom in The Tale of Melibee, one of the 24 stories narrated in The Canterbury Tales:
This is to seyn, that thee is bettre holde thy tonge stille than for to speke
(This is to say, that for thee it is better hold thy tongue still than to speak)